Single-Domain Antibodies for the Detection of SARS-CoV-2 Nucleocapsid Protein.

The goal of this work was to develop recombinantly expressed variable domains derived from camelid heavy-chain antibodies known as single-domain antibodies (sdAbs) directed against the SARS-CoV-2 nucleocapsid protein for incorporation into detection assays. To achieve this, a llama was immunized using a recombinant SARS-CoV-2 nucleocapsid protein and an immune phage-display library of variable domains was developed. The sdAbs selected from this library segregated into five distinct sequence families. Three of these families bind to unique epitopes with high affinity, low nM to sub-nM KD, as determined by surface plasmon resonance. To further enhance the utility of these sdAbs for the detection of nucleocapsid protein, homobivalent and heterobivalent genetic fusion constructs of the three high-affinity sdAbs were prepared. The effectiveness of the sdAbs for the detection of nucleocapsid protein was evaluated using MagPlex fluid array assays, a multiplexed immunoassay on color-coded magnetic microspheres. Using the optimal bivalent pair, one immobilized on the microsphere and the other serving as the biotinylated recognition reagent, a detection limit as low as 50 pg/mL of recombinant nucleocapsid and of killed virus down to 1.28 × 103 pfu/mL was achieved. The sdAbs described here represent immune reagents that can be tailored to be optimized for a number of detection platforms and may one day aid in the detection of SARS-CoV-2 to assist in controlling the current pandemic.

Oriented Immobilization of Single-Domain Antibodies Using SpyTag/SpyCatcher Yields Improved Limits of Detection.

Single-domain antibodies (sdAb), recombinantly produced variable heavy domains derived from the unconventional heavy chain antibodies found in camelids, provide stable, well-expressed binding elements with excellent affinity that can be tailored for specific applications through protein engineering. Complex matrices, such as plasma and serum, can dramatically reduce assay sensitivity. Thus, to achieve highly sensitive detection in complex matrices a highly efficient assay is essential. We produced sdAb as genetically linked dimers, and trimers, each including SpyTag at their C-terminus. The constructs were immobilized onto dyed magnetic microspheres to which SpyCatcher had been coupled and characterized in terms of their performance as capture reagents in sandwich assays. Initial tests on the ability of oriented monomer, dimer, and trimer captures to improve detection versus unoriented constructs in an assay for staphylococcal enterotoxin B spiked into buffer showed the oriented dimer format provided the best sensitivity while offering robust protein production. Thus, this format was utilized to improve a sdAb-based assay for the detection of dengue virus (DENV) nonstructural protein 1 (NS1) in serum. Detection of NS1 from each of the four DENV serotypes spiked into 50% normal human serum was increased by at least a factor of 5 when using the oriented dimer capture. We then demonstrated the potential of using the oriented dimer capture to improve detection of NS1 in clinical samples. This general method should enhance the utility of sdAb incorporated into any diagnostic assay, including those for high consequence pathogens.

Selection, characterization, and thermal stabilization of llama single domain antibodies towards Ebola virus glycoprotein.

BACKGROUND: A key advantage of recombinant antibody technology is the ability to optimize and tailor reagents. Single domain antibodies (sdAbs), the recombinantly produced variable domains derived from camelid and shark heavy chain antibodies, provide advantages of stability and solubility and can be further engineered to enhance their properties. In this study, we generated sdAbs specific for Ebola virus envelope glycoprotein (GP) and increased their stability to expand their utility for use in austere locals. Ebola virus is extremely virulent and causes fatal hemorrhagic fever in ~ 50 percent of the cases. The viral GP binds to host cell receptors to facilitate viral entry and thus plays a critical role in pathogenicity. RESULTS: An immune phage display library containing more than 107 unique clones was developed from a llama immunized with a combination of killed Ebola virus and recombinantly produced GP. We panned the library to obtain GP binding sdAbs and isolated sdAbs from 5 distinct sequence families. Three GP binders with dissociation constants ranging from ~ 2 to 20 nM, and melting temperatures from ~ 57 to 72 °C were selected for protein engineering in order to increase their stability through a combination of consensus sequence mutagenesis and the addition of a non-canonical disulfide bond. These changes served to increase the melting temperatures of the sdAbs by 15-17 °C. In addition, fusion of a short positively charged tail to the C-terminus which provided ideal sites for the chemical modification of these sdAbs resulted in improved limits of detection of GP and Ebola virus like particles while serving as tracer antibodies. CONCLUSIONS: SdAbs specific for Ebola GP were selected and their stability and functionality were improved utilizing protein engineering. Thermal stability of antibody reagents may be of particular importance when operating in austere locations that lack reliable refrigeration. Future efforts can evaluate the potential of these isolated sdAbs as candidates for diagnostic or therapeutic applications for Ebola.

Viral nanoparticle-encapsidated enzyme and restructured DNA for cell delivery and gene expression.

Packaging specific exogenous active proteins and DNAs together within a single viral-nanocontainer is challenging. The bacteriophage T4 capsid (100 × 70 nm) is well suited for this purpose, because it can hold a single long DNA or multiple short pieces of DNA up to 170 kb packed together with more than 1,000 protein molecules. Any linear DNA can be packaged in vitro into purified procapsids. The capsid-targeting sequence (CTS) directs virtually any protein into the procapsid. Procapsids are assembled with specific CTS-directed exogenous proteins that are encapsidated before the DNA. The capsid also can display on its surface high-affinity eukaryotic cell-binding peptides or proteins that are in fusion with small outer capsid and head outer capsid surface-decoration proteins that can be added in vivo or in vitro. In this study, we demonstrate that the site-specific recombinase cyclic recombination (Cre) targeted into the procapsid is enzymatically active within the procapsid and recircularizes linear plasmid DNA containing two terminal loxP recognition sites when packaged in vitro. mCherry expression driven by a cytomegalovirus promoter in the capsid containing Cre-circularized DNA is enhanced over linear DNA, as shown in recipient eukaryotic cells. The efficient and specific packaging into capsids and the unpackaging of both DNA and protein with release of the enzymatically altered protein-DNA complexes from the nanoparticles into cells have potential in numerous downstream drug and gene therapeutic applications.

Enhanced production of a single domain antibody with an engineered stabilizing extra disulfide bond.

BACKGROUND: Single domain antibodies derived from the variable region of the unique heavy chain antibodies found in camelids yield high affinity and regenerable recognition elements. Adding an additional disulfide bond that bridges framework regions is a proven method to increase their melting temperature, however often at the expense of protein production. To fulfill their full potential it is essential to achieve robust protein production of these stable binding elements. In this work, we tested the hypothesis that decreasing the isoelectric point of single domain antibody extra disulfide bond mutants whose production fell due to the incorporation of the extra disulfide bond would lead to recovery of the protein yield, while maintaining the favorable melting temperature and affinity. RESULTS: Introduction of negative charges into a disulfide bond mutant of a single domain antibody specific for the L1 antigen of the vaccinia virus led to approximately 3.5-fold increase of protein production to 14 mg/L, while affinity and melting temperature was maintained. In addition, refolding following heat denaturation improved from 15 to 70 %. It also maintained nearly 100 % of its binding function after heating to 85 °C for an hour at 1 mg/mL. Disappointingly, the replacement of neutral or positively charged amino acids with negatively charged ones to lower the isoelectric point of two anti-toxin single domain antibodies stabilized with a second disulfide bond yielded only slight increases in protein production. Nonetheless, for one of these binders the charge change itself stabilized the structure equivalent to disulfide bond addition, thus providing an alternative route to stabilization which is not accompanied by loss in production. CONCLUSION: The ability to produce high affinity, stable single domain antibodies is critical for their utility. While the addition of a second disulfide bond is a proven method for enhancing stability of single domain antibodies, it frequently comes at the cost of reduced yields. While decreasing the isoelectric point of double disulfide mutants of single domain antibodies may improve protein production, charge addition appears to consistently improve refolding and some charge changes can also improve thermal stability, thus providing a number of benefits making the examination of such mutations worth consideration.

Negative tail fusions can improve ruggedness of single domain antibodies.

Single-domain antibodies (sdAbs), the recombinantly expressed binding domains derived from the heavy-chain-only antibodies found in camelids and sharks, are valued for their ability to refold after heat denaturation. However, some sdAbs are prone to aggregation on extended heating at high concentration. Additionally, sdAbs prepared cytoplasmically often lack the conserved disulfide bond found in variable heavy domains, which both decreases their melting point and can decrease their ability to refold. Genetic fusions of sdAbs with the acid tail of α-synuclein (ATS) resulted in constructs that had enhanced ability to resist aggregation. In addition, almost complete refolding was observed even in the absence of the disulfide bond. These sdAb-ATS fusions expand the utility of sdAbs. They provide sdAbs that are resistant to aggregation, and enable the production of re-foldable sdAbs in the reducing environment of the cytoplasm.

Deletion of the Hoc and Soc capsid proteins affects the surface and cellular uptake properties of bacteriophage T4 derived nanoparticles.

Recently the use of engineered viral scaffolds in biotechnology and medical applications has been increasing dramatically. T4 phage capsid derived nanoparticles (NPs) have potential advantages as sensors and in biotechnology. These applications require that the physical properties and cellular uptake of these NPs be understood. In this study we used a T4 deletion mutant to investigate the effects of removing both the Hoc and Soc proteins from the capsid surface on T4 tailless NPs. The surface charge, zeta potential, size, and cellular uptake efficiencies for both the T4 NP and T4ΔHocΔSoc NP mutant were measured and compared using dynamic light scattering and flow cytometry and significant differences were detected.

Integrating Single Domain Antibodies into Field-Deployable Rapid Assays.

Single domain antibodies (sdAb) are the recombinant variable heavy domains derived from camelid heavy-chain antibodies. While they have binding affinities equivalent to conventional antibodies, sdAb are only one-tenth the size and possess numerous advantages such as excellent thermal stability with the ability to refold following denaturation, and inexpensive production in Escherichia coli or yeast. However, their small size does have drawbacks, one being that they can lose activity upon attachment or adsorption to surfaces, or may fail to adsorb efficiently, as they are highly soluble. This can make the transition from using conventional antibodies to sdAb nontrivial for assay development. Specifically, it is often necessary to re-optimize the protocols and tailor the recombinant sdAb through protein engineering to function efficiently in handheld assays, which currently are utilized for point of care testing and field applications. This work focuses on optimizing the integration of sdAb into rapid vertical flow assays. To achieve this goal, we engineered sdAb-based constructs and developed general protocols for the attachment of the sdAb to both gold nanoparticles and a support membrane. We achieved a limit of detection of 0.11 µg/mL for toxins staphylococcal enterotoxin B and ricin, both potential biothreat agents. Additionally, we demonstrated the ability to detect the nucleocapsid protein of SARS-CoV-2, a common target of antigen tests for COVID-19.

Bivalent single domain antibody constructs for effective neutralization of Venezuelan equine encephalitis.

Venezuelan equine encephalitis virus (VEEV) is a mosquito borne alphavirus which leads to high viremia in equines followed by lethal encephalitis and lateral spread to humans. In addition to naturally occurring outbreaks, VEEV is a potential biothreat agent with no approved human vaccine or therapeutic currently available. Single domain antibodies (sdAb), also known as nanobodies, have the potential to be effective therapeutic agents. Using an immune phage display library derived from a llama immunized with an equine vaccine that included inactivated VEEV, five sdAb sequence families were identified that showed varying ability to neutralize VEEV. One of the sequence families had been identified previously in selections against chikungunya virus, a related alphavirus of public health concern. A key advantage of sdAb is the ability to optimize properties such as neutralization capacity through protein engineering. Neutralization of VEEV was improved by two orders of magnitude by genetically linking sdAb. One of the bivalent constructs showed effective neutralization of both VEEV and chikungunya virus. Several of the bivalent constructs neutralized VEEV in cell-based assays with reductions in the number of plaques by 50% at protein concentrations of 1 ng/mL or lower, making future evaluation of their therapeutic potential compelling.

Isolation of a highly thermal stable lama single domain antibody specific for Staphylococcus aureus enterotoxin B.

BACKGROUND: Camelids and sharks possess a unique subclass of antibodies comprised of only heavy chains. The antigen binding fragments of these unique antibodies can be cloned and expressed as single domain antibodies (sdAbs). The ability of these small antigen-binding molecules to refold after heating to achieve their original structure, as well as their diminutive size, makes them attractive candidates for diagnostic assays. RESULTS: Here we describe the isolation of an sdAb against Staphyloccocus aureus enterotoxin B (SEB). The clone, A3, was found to have high affinity (Kd = 75 pM) and good specificity for SEB, showing no cross reactivity to related molecules such as Staphylococcal enterotoxin A (SEA), Staphylococcal enterotoxin D (SED), and Shiga toxin. Most remarkably, this anti-SEB sdAb had an extremely high Tm of 85°C and an ability to refold after heating to 95°C. The sharp Tm determined by circular dichroism, was found to contrast with the gradual decrease observed in intrinsic fluorescence. We demonstrated the utility of this sdAb as a capture and detector molecule in Luminex based assays providing limits of detection (LODs) of at least 64 pg/mL. CONCLUSION: The anti-SEB sdAb A3 was found to have a high affinity and an extraordinarily high Tm and could still refold to recover activity after heat denaturation. This combination of heat resilience and strong, specific binding make this sdAb a good candidate for use in antibody-based toxin detection technologies.

Engineered T4 viral nanoparticles for cellular imaging and flow cytometry.

Viruses are of particular interest as scaffolds for biotechnology applications given their wide range of shapes and sizes and the possibility to modify them with a variety of functional moieties to produce useful virus-based nanoparticles (VNPs). In order to develop functional VNPs for cell imaging and flow cytometry applications, we used the head of the T4 bacteriophage as a scaffold for bioconjugation of fluorescent dyes. Bacteriophage T4 is a double-stranded DNA virus with an elongated icosahedron head and a contractile tail. The head is ∼100 nm in length and ∼90 nm in width. The large surface area of the T4 head is an important advantage for the development of functional materials since it can accommodate significantly larger numbers of functional groups, such as fluorescent dyes, in comparison with other VNPs. In this study, Cy3 and Alexa Fluor 546 were chemically incorporated into tail-less T4 heads (T4 nanoparticles) for the first time, and the fluorescent properties of the dye-conjugated nanoparticles were characterized. The T4 nanoparticles were labeled with up to 19 000 dyes, and in particular, the use of Cy3 led to fluorescent enhancements of up to 90% compared to free Cy3. We also demonstrate that the dye-conjugated T4 nanoparticles are structurally stable and that they can be used as molecular probes for cell imaging and flow cytometry applications.

Immunodiagnostic reagents using llama single domain antibody-alkaline phosphatase fusion proteins.

Naive libraries of single domain antibodies (sdAbs) enable rapid isolation of binders to nearly any target. These binders, however, lack the benefits bestowed by in vivo affinity maturation and typically have low affinity toward their targets. We expressed five low-affinity toxin binding sdAbs, previously selected from a naive library derived from variable regions of llama heavy chain-only antibodies, as fusions with a hyperactive mutant Escherichia coli alkaline phosphatase (AP) and examined the impact on apparent affinity and utility. AP spontaneously dimerizes in solution, effectively dimerizing the fused sdAbs, imparting avidity in place of the lower affinity monomeric interactions. The sdAb-AP fusion also combines the target recognition domain with a signal transduction domain, commonly used in enzyme-linked immunosorbent assays (ELISAs). The functional affinity of the sdAb-AP fusions, often increased by a factor of 10 over unfused sdAbs, and their utility as tracer reagents in ELISAs was dramatically improved, giving limits of detection of 300 ng/ml or less, whereas parental sdAbs gave no discernible signal at the toxin concentrations tested. The fusion of sdAbs to AP presents a valuable route to facilitate the implementation of sdAb-based immunoreagents rapidly selected from existing naive libraries toward new or emerging threats.

The SARS-CoV-2 SSHHPS Recognized by the Papain-like Protease.

Viral proteases are highly specific and recognize conserved cleavage site sequences of ∼6-8 amino acids. Short stretches of homologous host-pathogen sequences (SSHHPS) can be found spanning the viral protease cleavage sites. We hypothesized that these sequences corresponded to specific host protein targets since >40 host proteins have been shown to be cleaved by Group IV viral proteases and one Group VI viral protease. Using PHI-BLAST and the viral protease cleavage site sequences, we searched the human proteome for host targets and analyzed the hit results. Although the polyprotein and host proteins related to the suppression of the innate immune responses may be the primary targets of these viral proteases, we identified other cleavable host proteins. These proteins appear to be related to the virus-induced phenotype associated with Group IV viruses, suggesting that information about viral pathogenesis may be extractable directly from the viral genome sequence. Here we identify sequences cleaved by the SARS-CoV-2 papain-like protease (PLpro) in vitro within human MYH7 and MYH6 (two cardiac myosins linked to several cardiomyopathies), FOXP3 (an X-linked Treg cell transcription factor), ErbB4 (HER4), and vitamin-K-dependent plasma protein S (PROS1), an anticoagulation protein that prevents blood clots. Zinc inhibited the cleavage of these host sequences in vitro. Other patterns emerged from multispecies sequence alignments of the cleavage sites, which may have implications for the selection of animal models and zoonosis. SSHHPS/nsP is an example of a sequence-specific post-translational silencing mechanism.

Stabilization of a Broadly Neutralizing Anti-Chikungunya Virus Single Domain Antibody.

A single domain antibody (clone CC3) previously found to neutralize a vaccine strain of the chikungunya virus (PRNT50 = 2. 5 ng/mL) was found to be broadly neutralizing. Clone CC3 is not only able to neutralize a wild-type (WT) strain of chikungunya virus (CHIKV), but also neutralizes WT strains of Mayaro virus (MAYV) and Ross River virus (RRV); both arthralgic, Old World alphaviruses. Interestingly, CC3 also demonstrated a degree of neutralizing activity against the New World alphavirus, Venezuelan equine encephalitis virus (VEEV); albeit both the vaccine strain, TC-83, and the parental, WT Trinidad donkey strain had PRNT50 values ~1,000-fold higher than that of CHIKV. However, no neutralization activity was observed with Western equine encephalitis virus (WEEV). Ten CC3 variants designed to possess a range of isoelectric points, both higher and lower, were constructed. This approach successfully identified several lower pI mutants which possessed improved thermal stabilities by as much as 10°C over the original CC3 (Tm = 62°C), and excellent refolding abilities while maintaining their capacity to bind and neutralize CHIKV.

Llama-derived single-domain antibodies for the detection of botulinum A neurotoxin.

Single-domain antibodies (sdAb) specific for botulinum neurotoxin serotype A (BoNT A) were selected from an immune llama phage display library derived from a llama that was immunized with BoNT A toxoid. The constructed phage library was panned using two methods: panning on plates coated with BoNT A toxoid (BoNT A Td) and BoNT A complex toxoid (BoNT Ac Td) and panning on microspheres coupled to BoNT A Td and BoNT A toxin (BoNT A Tx). Both panning methods selected for binders that had identical sequences, suggesting that panning on toxoided material may be as effective as panning on bead-immobilized toxin for isolating specific binders. All of the isolated binders tested were observed to recognize bead-immobilized BoNT A Tx in direct binding assays, and showed very little cross-reactivity towards other BoNT serotypes and unrelated protein. Sandwich assays that incorporated selected sdAb as capture and tracer elements demonstrated that all of the sdAb were able to recognize soluble ("live") BoNT A Tx and BoNT Ac Tx with virtually no cross-reactivity with other BoNT serotypes. The isolated sdAb did not exhibit the high degree of thermal stability often associated with these reagents; after the first heating cycle most of the binding activity was lost, but the portion of the protein that did refold and recover antigen-binding activity showed only minimal loss on subsequent heating and cooling cycles. The binding kinetics of selected binders, assessed by both an equilibrium fluid array assay as well as surface plasmon resonance (SPR) using toxoided material, gave dissociation constants (K(D)) in the range 2.2 x 10(-11) to 1.6 x 10(-10) M. These high-affinity binders may prove beneficial to the development of recombinant reagents for the rapid detection of BoNT A, particularly in field screening and monitoring applications.

Multi-Enzyme Assembly on T4 Phage Scaffold.

Over the past two decades, various scaffolds have been designed and synthesized to organize enzyme cascades spatially for enhanced enzyme activity based on the concepts of substrate channeling and enhanced stability. The most bio-compatible synthetic scaffolds known for enzyme immobilization are protein and DNA nanostructures. Herein, we examined the utility of the T4 phage capsid to serve as a naturally occurring protein scaffold for the immobilization of a three-enzyme cascade: Amylase, Maltase, and Glucokinase. Covalent constructs between each of the enzymes and the outer capsid protein Hoc were prepared through SpyTag-SpyCatcher pairing and assembled onto phage capsids in vitro with an estimated average of 90 copies per capsid. The capsid-immobilized Maltase has a fourfold higher initial rate relative to Maltase free in solution. Kinetic analysis also revealed that the immobilized three-enzyme cascade has an 18-fold higher converted number of NAD+ to NADH relative to the mixtures in solution. Our results demonstrate that the T4 phage capsid can act as a naturally occurring scaffold with substantial potential to enhance enzyme activity by spatially organizing enzymes on the capsid Hoc.

Selection and Characterization of Single-Domain Antibodies for Detection of Lassa Nucleoprotein.

Lassa virus is the etiologic agent of Lassa fever, an acute and often fatal illness endemic to West Africa. It is important to develop new reagents applicable either for the specific diagnosis or as improved therapeutics for the treatment of Lassa fever. Here, we describe the development and initial testing of llama-derived single-domain antibodies that are specific for the Lassa virus nucleoprotein. Four sequence families based on complementarity-determining region (CDR) homology were identified by phage-based enzyme-linked immunosorbent assays, however, the highest affinity clones all belonged to the same sequence family which possess a second disulfide bond between Framework 2 and CDR3. The affinity and thermal stability were evaluated for each clone. A MagPlex-based homogeneous sandwich immunoassay for Lassa virus-like particles was also demonstrated to show their potential for further development as diagnostic reagents.

Lipid-tagged single domain antibodies for improved enzyme-linked immunosorbent assays.

Anti-Staphylococcal Enterotoxin B single domain antibodies were engineered to include the N-terminal peptide sequence of the major outer membrane lipoprotein from Escherichia coli, which directs the N-terminal addition of lipid to the single domain antibody. We produced and purified two different single domain antibodies as well as a variant and dimer construct of one of the two, all with and without the added lipid. Their ability to function as the capture antibody in standard enzyme-linked immunosorbent assays were evaluated, finding that coating polystyrene microtiter plates with the lipid-tagged single domain antibodies gave a 3-fold improvement in the observed limit of detection. This increase was likely due to an increased amount of single domain antibody adsorbed to the microtiter plate, which translated to improved limits of detection of Staphylococcal Enterotoxin B over using the same single domain antibody sans lipid-tag. However, improved orientation may also play a role. Regardless of the mechanism, the biosynthetic lipid-tagging of single domain antibodies represent a facile modality that can enhance their ability to be utilized as immunoassay capture reagent as well as facilitate their incorporation into liposome targeting applications in the future.

Bacteriophage t4 nanoparticles as materials in sensor applications: variables that influence their organization and assembly on surfaces.

Bacteriophage T4 nanoparticles possess characteristics that make them ideal candidates as materials for sensors, particularly as sensor probes. Their surface can be modified, either through genetic engineering or direct chemical conjugation to display functional moieties such as antibodies or other proteins to recognize a specific target. However, in order for T4 nanoparticles to be utilized as a sensor probe, it is necessary to understand and control the variables that determine their assembly and organization on a surface. The aim of this work is to discuss some of variables that we have identified as influencing the behavior of T4 nanoparticles on surfaces. The effect of pH, ionic strength, substrate characteristics, nanoparticle concentration and charge was addressed qualitatively using atomic force microscopy (AFM).

Ricin detection using phage displayed single domain antibodies.

Phage-displayed single domain antibodies (sdAb) were compared to monomeric solubly expressed sdAb and llama polyclonal antibodies for the detection of ricin. SdAb are comprised of the variable domain derived from camelid heavy chain only antibodies (HcAb). Although HcAb lack variable light chains, they as well as their derivative sdAb are able to bind antigens with high affinity. The small size of sdAb (∼16 kDa), while advantageous in many respects, limits the number of labels that can be incorporated. The ability to incorporate multiple labels is a beneficial attribute for reporter elements. Opportunely, sdAb are often selected using phage display methodology. Using sdAb displayed on bacteriophage M13 as the reporter element gives the potential for incorporating a very high number of labels. We have demonstrated the use of both sdAb and phage- displayed sdAb for the detection of ricin using both enzyme linked immunosorbent assays (ELISAs) and Luminex fluid array assays. The phage-displayed sdAb led to five to ten fold better detection of ricin in both the ELISA and Luminex assays, resulting in limits of detection of 1 ng/mL and 64 pg/mL respectively. The phage-displayed sdAb were also dramatically more effective for the visualization of binding to target in nitrocellulose dot blot assays, a method frequently used for epitope mapping.